Methods to increase the number of filters per optical path in a downhole spectrometer
Downhole spectrometer tools are provided with two ways to increase the number of filters on an optical path. A first approach employs multiple filter wheels that rotate alternately in a common plane to intersect the optical path. Portions of the wheels are cut out to avoid mechanical interference between the wheels. A second approach drives the one or more filter wheels with a wobble that causes the filters to trace one or more hypocycloidal curves that each intersect the optical path.
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Engineers use downhole spectrometers to monitor, analyze, or identify different properties of fluid, such as contamination, composition, fluid type, and PVT (“pressure, volume, temperature”) properties. For example, a spectrometer may be coupled to a formation fluid sampling tool to analyze fluids in real time as they are drawn from the formation. During the sampling operation the spectrometer can monitor contamination levels from borehole fluids and, once the contamination has fallen to an acceptable level, the spectrometer can measure spectral characteristics of the formation fluid to identify its components. Fluid component identification is helpful for determining whether and how production should be performed from a particular area of the well. It can provide indications of reservoir continuity, blowout risk, production value, etc.
Despite the evident utility of downhole spectrometers, the range of measurements that can be made by existing tools is somewhat limited. In the case of filter wheel spectrometers, this limitation is primarily due to spatial constraints on the filter wheel itself.
BRIEF DESCRIPTION OF THE DRAWINGS
A hypocycloid is sometimes defined as a shape drawn out by a fixed point on a small circle as it rotates inside a larger circle. However, as the term “hypocycloid” is used herein, it includes the shapes drawn by any point fixed relative to a first circle as it rotates inside or outside a second circle of smaller or larger diameter. Specific examples of such shapes elsewhere termed epicycloids, epitrochoids, and hypotrochoids, are included within the scope of this term as used in the present specification and claims.
The term “fluid” as used herein includes both liquids and gases.
The issues identified in the background are at least in part addressed by the disclosed methods for increasing the number of filters per optical path in a downhole spectrometer. Embodiments of a first method employ a set of filter wheels in a common plane that intersects an optical path. Each of the wheels is provided with a shape that permits rotation of the individual wheels without mechanically interfering with the other wheel(s) in the set. Embodiments of a second method employ a drive mechanism that causes points on the filter wheel to trace out hypocycloidal paths. The filters in the wheel are arranged so that their corresponding paths each intersect with the optical path. Some downhole spectrometer tools may employ both methods so that of multiple filter wheels is driven with the hypocycloidal drive mechanism.
To further assist the reader's understanding of the disclosed systems and methods, we describe a suitable environment for their use and operation. Accordingly,
A downhole optical fluid analyzer can be employed to characterize downhole fluids in both of the foregoing logging environments. For example,
Aside from optional calibration elements such as an open aperture or a fully opaque light stop, the filters 409 are chosen to measure particular spectral characteristics suitable for identifying or otherwise characterizing the contents of the sample cell. As such, the filters may include bandpass filters, bandstop filters, and multivariate optical elements (MOE). The intensity of the light striking the detector is thus a measure of some portion of the spectral fingerprint mentioned previously. To ensure an adequate signal-to-noise ratio, the filters must be larger or equal to some given size that is a function of the manufacturing specifications for the other components (e.g., light source intensity, detector sensitivity, wheel rotation rate, and sample cell size). Moreover, the filter wheel has a limited circumference within which the filters must be placed, thereby limiting the number of filters that can be positioned in a given wheel.
To address this limitation, at least some of the disclosed downhole tool embodiments employ multiple filter wheels. Because the tool design generally requires that all of the filters intercept the optical path at a given position, the multiple filter wheels are located in a common plane as illustrated in
Existing filter wheel designs for downhole optical fluid analyzers employ a filter wheel diameter of 3.188 inches which is sufficient to hold 20 filters, of which one may be an open aperture for calibration. It is expected that the omitted area for filter wheels in the two-wheel design will reduce the number of filters around the circumference to 17, thereby increasing the total number of filters to 34. Calibration can be performed when both wheels are clear from the optical path, eliminating the need for a calibration aperture in one of the filter positions. In this case, the two wheel design yields a 79% increase in the number of usable filters, without requiring the use of a beam splitter or optical switch that would decrease light intensity and/or reduce the tool's reliability in a downhole environment.
The two-wheel design can be extended to employ three or more filter wheels, each having an omitted segment to enable the rotation of each of the other wheels. As the number of filter wheels grows, so too does the size of each wheel's omitted segment, thereby limiting the amount of gains that can be made in this way.
A number of mechanisms (represented by rotation mechanism options block 510) may be employed to rotate the filter wheels 502. Some embodiments employ a separate electric motor is provided to drive each filter wheel. While having an advantage of implementation ease, it is expected that powering electrical motors in an on/off fashion reduces battery life and reduces reliability of the tool. Accordingly, a continuously-running electrical motor may be employed with two clutches to drive the wheels in alternation. Alternatively, a cam assembly or intermittent drive mechanism (such as a variant of a Geneva drive) can be employed to convert the continuous motion of the electrical motor into alternate rotations of the wheels.
Another way to increase the number of filters in a downhole spectrometer is to employ a filter wheel drive mechanism that causes points on the filter wheel to trace out hypocycloidal paths. Because the filter wheel “wobbles”, the curve traced on the wheel by the optical path has a substantially greater length than the circumference of the wheel, thereby enabling the use of a greater number of filters on one wheel than would otherwise be possible.
Other ratios and wheel configurations can be employed to vary the number and size of lobes in the hypocycloidal curves traced out by the filters. In each case, the wheel's wobble enables filters placed at multiple radial distances from the wheel's axis to still pass through the optical path. Moreover, the hypocycloidal drive mechanism can be employed for each of multiple filter wheels in a common plane so as to further increase the number of filters in the downhole spectrometer.
Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. For example, the filter elements can be either transmissive or reflective filters, and the filter wheels can precede or follow the sample cell. It is intended that the following claims be interpreted to embrace all such variations and modifications.
1. A method for increasing the number of optical filters in a downhole spectrometer, the method comprising:
- providing a set of filter wheels on a common plane to intersect an optical path, wherein each of said wheels has an omitted segment that provides a clearance that permits rotation of individual wheels without interference; and
- rotating said individual wheels in turn to pass filters from each wheel across said optical path, said rotating causing a filter from a first wheel of the set to trace a first path that intersects, within said common plane, a second path traced by a filter from a second wheel of the set.
2. The method of claim 1, wherein said set of filter wheels has no more than two wheels.
3. The method of claim 1, wherein said rotating employs a separate motor for each wheel.
4. The method of claim 1, wherein said rotating employs a motor with a clutch for each wheel.
5. The method of claim 1, wherein said rotating employs a mechanism that converts continuous motion into intermittent motion.
6. The method of claim 1, wherein said rotating employs a cam system to rotate the wheels in alternation.
7. The method of claim 1, further comprising passing light through a downhole sample cell on the optical path.
8. The method of claim 1, wherein the rotations of said individual wheels in turn occur without transverse movement of the wheels in between the rotations.
9. A method for increasing the number of optical filters in a downhole spectrometer, the method comprising:
- driving an inner filter wheel with a mechanism that causes individual points to trace out hypocycloidal paths as a result of concurrent rotation of the inner filter wheel and an outer wheel having a diameter larger than a diameter of the inner filter wheel; and
- arranging filters on the inner filter wheel so that their corresponding hypocycloidal paths each intersect each other on an optical path.
10. The method of claim 9, wherein said filters are at two or more radial distances from a center of the inner filter wheel.
11. The method of claim 9, wherein the hypocycloidal paths are epicycloidal.
12. The method of claim 9, wherein the hypocycloidal paths are epitrochoids.
13. The method of claim 9, wherein the filters trace out no more than two hypocycloidal paths.
14. The method of claim 9, wherein the filters trace out at least three hypocycloidal paths.
15. The method of claim 9, further comprising passing light through a downhole sample cell on the optical path.
16. The method of claim 9, further comprising driving said filter wheel with a motion that causes said filters to each cross the optical path.
17. A downhole spectrometer tool that comprises:
- a downhole sample cell having a fluid sample;
- a filter wheel that turns around an inner or outer circumference of an outer gear to move its filters along hypocycloidal curves as a result of concurrent rotation of the filter wheel and the outer gear, the outer gear having a diameter larger than a diameter of the filter wheel; and
- an optical path through the sample cell and an intersection of the hypocycloidal curves.
18. The tool of claim 17, wherein the filter wheel has filters at two or more radial distances from its center.
19. The tool of claim 17, wherein said curves are epicycloids, epitrochoids, or hypotrochoids.
20. The tool of claim 17, wherein the filter wheel moves its filters along no more than two curves.
21. The tool of claim 17, wherein the filter wheel moves its filters along at least three curves.
22. The tool of claim 17, further comprising a second filter wheel, wherein both filter wheels rotate through the optical path on a common plane.
23. A downhole spectrometer tool that comprises:
- a downhole sample cell having a fluid sample;
- a set of filter wheels on a common plane to intersect an optical path, wherein each of said wheels has an omitted segment that provides a clearance that permits rotation of individual wheels without interference; and
- a rotation mechanism that rotates the individual wheels in turn to pass filters from each wheel across said optical path, wherein the rotation mechanism causes a filter from a first wheel of the set to trace a first path that intersects, within said common plane, a second path traced by a filter from a second wheel of the set.
24. The tool of claim 23, wherein the rotation mechanism comprises a separate motor for each wheel.
25. The tool of claim 23, wherein the rotation mechanism comprises a motor with a clutch for each wheel.
26. The tool of claim 23, wherein the rotation mechanism converts continuous motion into intermittent motion.
27. The tool of claim 23, wherein the rotation mechanism comprises a cam system to rotate the wheels in alternation.
28. The tool of claim 23, wherein the rotation mechanism causes each wheel of the set to complete a full rotation while other wheels of the set are held.
29. The tool of claim 23, wherein the rotation mechanism pauses as each filter enters the optical path.
30. The tool of claim 23, wherein each individual wheel of the set includes a plurality of spaced filters around its circumference.
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Foreign Patent Documents
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Filed: May 24, 2011
Date of Patent: May 31, 2016
Patent Publication Number: 20140070083
Assignee: HALLIBURTON ENERGY SERVICES, INC. (Houston, TX)
Inventors: Wei Zhang (Houston, TX), Robert Atkinson (Richmond, TX), Michael T. Pelletier (Houston, TX), Christopher M. Jones (Houston, TX)
Primary Examiner: Kiho Kim
Application Number: 14/117,542
International Classification: G01V 5/00 (20060101); G01N 21/25 (20060101); G01J 3/12 (20060101); G01J 3/51 (20060101); G02B 26/00 (20060101); G01V 8/00 (20060101); G01J 3/02 (20060101);